Barcelona and Napoli now have their own problems. For the decline I see Napoli as having the worse decline, we can see that in Serie A. To be honest, I always use the domestic league as a measure, even though it's a different competition, but in the domestic league they have a schedule every week, so we can look at them and judge every week, whether they have decreased or improved.
I agree with you that it won't be easy to predict this match, and it's possible for them to beat each other and it will be their goal productivity that decides it. Both Napoli and Barcelona will have to watch out for each other.
Napoli have more complicated problem than Barcelona regarding in domestic league performance very poor, Napoli losses 3 of 5 matches in domestic league and eliminated from Copa Italia make they have completed bad result before facing Barcelona on knock out round. Barcelona performance in domestic league not really bad such as Napoli because the losses 1 of 5 matches before. Indeed both teams have poor performance difference when playing in Champion League, not easy for predicting between Napoli or Barcelona have large chance for winning the match because Champion league will give difference atmosphere for both team get chance qualify to quarter final round.
Get improving performance for both teams in domestic league and when facing in Champion league next month later have quit well performance and show us more interested match.